1. Field of the Invention
The present invention relates to systems and methods for mooring vessels offshore. Particularly, the present invention relates to systems and methods for mooring vessels which will be expected to remain on station for extended periods of time, or perhaps permanently, under weather conditions which may include heavy seas, high winds and strong currents. More particularly, the present invention relates to systems and methods for mooring floating ships, barges and semi submersible vessels employed as drilling vessels, tenders, oil production vessels and oil storage vessels and barges.
2. Background of the Invention
A mooring system for vessels intended to be moored offshore is a large, expensive set of equipment comprising several thousand feet of mooring line comprising chains and wire lines, a windlass or winch with redundant braking systems for deploying, tensioning and retrieving the mooring line, fairleads for directing the mooring lines, line swivels to prevent kinks in the mooring chains, anchors (or alternatively a mooring piling) for providing holding power to keep the vessel moored on station, as well as a variety of specialized equipment to satisfy various mooring conditions. For each mooring point of a vessel, separate mooring tackle is required. The mooring line of each mooring tackle will be attached to an anchor, or, in some circumstances, to a piling driven into the bottom. Each ship will have a minimum of two mooring points, and some vessels such as drill ships, semi submersibles, tenders, oil production vessels and oil storage vessels, which are expected to remain on station for extended periods of time, may have eight to sixteen mooring points.
Mooring systems are designed to industry standards to ensure that the mooring systems provide the holding power required to withstand expected weather and sea conditions. Typically, mobile offshore vessels, such as drill ships and semi submersibles engaged in drilling operations, are designed to withstand maximum weather conditions expected during a 5-10 year period within their areas of expected operation. On the other hand, floating production vessels of similar size and type are designed for maximum weather conditions expected during a 50-100 year period in their area of intended operation. Consequently, floating production vessels are designed with two to three times the mooring strength of drilling vessels.
Increased attention to personnel safety and environmental protection is creating a trend toward requiring increased mooring strengths for mooring systems, particularly for vessels in offshore oil and gas drilling service. Additionally, with current economic conditions, operators often wish to employ drilling vessels, in areas having more severe weather conditions than conditions for which the vessel mooring systems were initially designed. Also, operators are finding it economically attractive to convert existing drilling vessels into production vessels which require much stronger mooring systems. In each of these circumstances means must be found for increasing the strength of the existing vessel mooring systems, or the existing mooring systems must be replaced with stronger mooring systems. Replacement of existing vessel mooring systems is doubly expensive. Not only must the new, stronger mooring systems be procured and installed, the existing mooring systems must be removed and scrapped.
Each mooring system for large vessels may cost in the millions of dollars. For vessels having eight or more mooring systems, mooring system may cost in the tens of millions of dollars.
3. Description of Pertinent Art
In the past, tender barges, designed for mooring along side fixed drilling and production platforms in the Gulf of Mexico, employed a plurality of mooring systems in order to hold the tenders on station adjacent the platforms with little lateral movement. The tenders employed up to eight mooring points for attachment to anchors, or very often, to permanently set anchor piling driven into the bottom. Each anchor or piling had a mooring line running upward and attached to a raft buoy at the water surface. Each raft buoy was large enough to hold its mooring line on the surface at all times, including times when a tender mooring line was attached and tensioned to moor the tender in place. A large sheave was mounted on each raft buoy. To moor a tender, a soft line was run out from the tender, through the sheave and back to the tender. This soft line was attached to a vessel mooring line which was then pulled out through the sheave and back to the tender where the end of the vessel mooring line was tied-off. This step was repeated for each sheave on each raft buoy. The vessel mooring lines were then taken-up and tensioned to position the tender alongside the platform. This mooring system was designed for shallow water and mild weather conditions. In heavy weather, the tender could slip its moorings for a tow to port by releasing the end of each vessel mooring line and pulling the vessel mooring lines back to the tender through the raft buoy sheaves.
This method for mooring tenders has several disadvantages which make it not useful for mooring larger vessels in deep waters or in heavy seas. The raft buoys, located at the water surface, were subject to wave action. In heavy seas, the raft buoys tended to lift the anchors, if anchors were used, or break the mooring lines if the anchors held to the sea bed, or if mooring piling were used instead of anchors. Additionally, the mooring lines from the tender to the raft buoys were at water surface level, subject to wave action, and were an obstruction to supply and service boats